Set up new package dgl_abm

[vanlankveldthijs]

When splitting the functionality in the repository into base ABM functionality and more specialized PTM functionality, we will want a new package to put the ABM functionality into.

This will be a mostly empty package, but should otherwise contain all the normal structure and supporting files.

[vmgaribay]

Update I met with Debraj today. We have reached a consensus on a wish list for DGL-ABM. It should have the capacity to:
initialize model agents and network, etc.
pass messages (what we do with trade_money, or similar)
weight update
random addition (like global_attachment, I believe)
link_deletion

Highly favored potential use case examples:
boltzmann_wealth_model_network
virus_on_network